Linking value to another anywhere in blender 2.8
1) add driver (ctrl d) to first value

2) copy driver (sorry im wrong do this at the end !!!!!)

3) paste driver to other value (sorry do this at the end !!!!!))

4) open a driver panel editor type, select the correct object otherwise the driver will not apear, also select the last step in the T panel on the left

5) go to n tab then driver tab

6) set a to scripted expression

7) the expression need to have the same name as the tab below "add input variable", so by default, "var"

8) next to "var" is a little symbol, click on it, set it to single proprety

9) create a custom proprety, in the propreties tab, you can create one in the scene tab, material tab, and object tab, and this custom proprety will appear on the N panel in the 3d viewport when the concerned object is selected, if you want the custom proprety to be not selection depentend then i advise you to create the proprety in the scene tab, or even world tab

10) we go back in the driver editor,in the n panel, and in our variable "prop" we choose the correct cathegory of where the custom proprety is

11) we need a path to the correct custom proprety we just created, so find him, right click on him, and "copy data path", then paste it in the "path" on the driver editor

12) we are done, tweak the custom proprety and the calue will change according to it, you may need to change the calue of the custom proprety in "edit" for it to correspond of the max and min value you want to tweak 



JUST COPY AND PASTE DRIVER AT THE END ! i did it at the beginning this may be wrong 



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in the case above its linking value for every single value possible in blender (from 0 to 1 at least)

but if you want to link value only trough the node, just use a nodegroup with an input value in it and paste it trough the materials.
in the future with everything node this will be really handy 



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